Searching for Transits

We search for extra-solar planets, or planets orbiting stars other than the Sun, by the transits they make. A transit occurs as a planet passes in front of its star as seen from the direction of Earth. Astronomers have long observed the planets Mercury and Venus transiting the Sun, but only recently has technology made it practical to observe transits by extra-solar planets of their stars.

There are differences between observing a transit of Venus or Mercury and observing the transit of an extra-solar planet. Other stars are too far away to be seen as anything but points of light in our telescopes. Consequently, we will have to wait for future advances in order to obtain direct images of a dark planet passing in front of its star, but we can measure the total amount of light coming from the star at any time. When we measure the a star's intensity during a planetary transit, we will see it diminish because some fraction of its light is blocked by the planet. This diminution may last several hours to several days. The record of a star's intensity versus time is called a light curve and we sketch the light curve of a transit in the figure below.

To detect the transit of an extra-solar planet, we observe the intensity of a star's light versus time and plot this behavior in as a light curve. As a planet transits across a star in the top row of figures, it blocks some of the star's light. During the transit, the intensity we observe decreases as sketched in the light curve on the bottom.

One of challenges faced by anyone hoping to discover an extra-solar planet by its transits is the fact that a transit lasts for only a small fraction of a planet's orbital period. Additionally, only a planet that orbits in a certain orientation relative to Earth will pass between its star and our telescope. This means that only a small fraction of extra-solar planets will ever make a transit and even for those that do, we need to observe its parent star for a total time comparable to the orbital period of the planet in order to have a good chance of observing a transit.

The solution to this dilemma is to simultaneously observe as many stars as possible and to monitor them (producing long light curves) for weeks at a time before turning our attention to another set of stars. If enough stars are observed, our chances to detect transiting planets are quite good. In order to make our observations, we use CCD cameras on telescopes to observe large patches of sky containing many stars, a few of which will hopefully harbor transiting planets. The same field of stars is observed every few minutes, all night, for several weeks. Our data consists of a large set of digital images, each of the same field of stars. The brightness of each star is measured on every image and this is the basis of our light curves.

The two pieces of information we will want to know right away is how big is the planet's orbit, and how big is the planet itself. To find both of these answers, we will need to know the size of the star the planet is orbiting. This is easily determined from a spectrum of the star.

For the duration of the orbit, the time the planet is in front of the star is related to the size of the orbit. The longer the planet is in front of the star, the bigger the orbit is.

The intensity of the drop tells the size of the planet, but again is dependant on star size. The percent drop is a simple ratio of the area of the star to the planet. A planet of a given size will produce a larger drop in light around a smaller star than a larger one. This is seen in the top part of the figure. At the same time, a set of different size planets orbiting around similar size stars will also give different drops in intensity. This is seen in the bottom part of the figure. Along with each image is a sample lightcurve demonstrating what would be seen in each case.

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